In wireless communication systems, Hybrid-Automatic Repeat Request (H-ARQ) may be utilized as a retransmission protocol to ensure accurate transmissions between a transmitter and a receiver, and to repeat transmissions that were unsuccessful or which contain errors in the transmitted data. Using H-ARQ, information received from a previous unsuccessful transmission of a packet is combined with the retransmitted data when that packet is retransmitted. The combined data packet is then attempted to be decoded by the receiver. In one version of H-ARQ, the same amount of information is retransmitted at each iteration until the receiver announces successful decoding of the packet, or until a maximum number of retransmissions is reached.
A variation of a H-ARQ process may be referred to as adaptive H-ARQ in which additional information from the receiver is fed back when the receiver reports to the transmitter of a packet failure. The receiver reports a packet failure via a negative acknowledgment (NACK) message. When the transmitter receives a NACK. message from the receiver, the retransmission parameters may be adjusted based on the additional information received along with the NACK message. Typically, such additional information comprises signal-to-noise ratio (SNR) infbrmation which is fed back to from the receiver to the transmitter. The transmitter then adjusts the retransmission parameters based on the SNR information of the previous transmission in an attempt to increase the likelihood that the retransmission of the packet will be more successful than the previous transmission attempt. In orthogonal frequency-division multiplexing (OFDM) systems, however, the SNR information may not be a sufficient indicator of the performance of the wireless system.
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